A desire of a cellular telephone user is to have a single cellular telephone that can operate anywhere in the world, a “world” phone. Unfortunately, in order to operate at different locations throughout the world, the world phone needs to be able to communicate with different cellular communications networks, each using a potentially different communications mechanism, as well as being able to tune its oscillator to different frequency ranges. This is due to the wide variety of communications standards being used in different nations. For example, in the United States, it is common to encounter GSM (Global System for Mobile Communications), CDMA (Code Division Multiple Access), and TDMA (Time-Division Multiple Access) cellular telephone networks. In addition to the three types of cellular telephone networks, there can be multiple commonly used frequency ranges, 850 and 900 MHz ranges as well as 1.8 and 1.9 GHz ranges. In other countries, other types of cellular telephone networks and frequency ranges may be in use.
One solution to the problem of being able to tune an oscillator to such a wide variety of frequency ranges is to have multiple oscillators, one for each frequency range of interest. With multiple oscillators, each oscillator can be optimized for each frequency range, potentially maximizing tuning accuracy.
Another solution to the problem is to use a single oscillator but with multiple oscillator cores or LC tanks. The multiple oscillator cores or LC tanks can be used to extend the tuning range of the single oscillator without needing multiple oscillators. Each of the multiple oscillator cores or LC tanks can be switched in when needed. Again, the use of multiple oscillator cores or LC tanks can allow the optimization for the different frequency ranges.
Yet another solution to the problem is to use a switch, such as a CMOS switch, to short circuit a portion of an inductor to increase the tuning range of the local oscillator. The use of the switch can permit the use of a single local oscillator.
One disadvantage of the prior art is that the use of multiple local oscillators and/or multiple oscillator cores or LC tanks is that the oscillators consume a considerable amount of silicon area. Therefore, it is desired to minimize the number of local oscillators or oscillator cores.
A second disadvantage of the prior art is that due to a high quality factor requirement for the inductor used in the oscillator, the power on resistance of the switch must be low. This places a requirement that the switch must be physically large, thereby requiring a large amount of silicon area to be dedicated to the switch and the oscillator. Furthermore, as the oscillator operating frequency increases, parasitic capacitance introduced by the large switch can become problematic due to the fact that the total capacitance of the oscillator core is small.